This current case-control study; as a first study, evaluated the association between dietary inflammatory index and hypothyroidism. Our findings showed that newly diagnosed hypothyroidism subjects had significantly higher pro-inflammatory diet scores in comparison with the control group.
These results approved our hypothesis that there is an association between a more pro-inflammatory diet (higher DII score) and an increased risk of hypothyroidism. Although there is no previous evidence that investigated the relationship between the DII and the risk of hypothyroidism, nevertheless our finding is in line with other studies [34–36], that examined the effect of the pro-inflammatory potential of diet on the development of other metabolic diseases. As the study by Paquet et al, has revealed that there is a significant association between higher DII scores; as reflected in the more pro-inflammatory diet, and a higher risk of differentiated thyroid carcinoma [34].
The DII has been previously shown to be associated with various inflammatory markers including hs-CRP, IL-6, TNF-a, and homocysteine [31, 37, 38]. The pro-inflammatory diet, as indicated by higher DII scores, is associated with higher systemic levels of inflammatory cytokines, while the anti-inflammatory diet, as indicated by lower DII scores, is accompanied by lower levels of these factors. The dietary inflammation index can predict the levels of various inflammatory markers [24, 31, 37]. Our findings showed a significant positive association between a higher pro-inflammatory score of DII and hypothyroidism, which was representative of the probable presence of systemic inflammation. This finding is in line with prior literature that reported elevated levels of CRP, TNF-a, and IL-6 in the early stage of hypothyroidism [19, 20, 39–43]. Recently, the possible role of this systematic inflammation in the pathogenesis of hypothyroidism has been proposed [21]. To this evidence, the pro-inflammatory diet can be proposed as a reason for emerging chronic inflammation in the pathogenesis of hypothyroidism.
Aging is also closely linked with raised systemic inflammatory markers such as CRP, IL-6, and TNF-α [44, 45]. Previous studies have shown that a higher pro-inflammatory diet (i.e. higher DII score) is associated with old age [46, 47]. Aging is known as a contributing factor to diet-induced inflammation [44, 45, 48]. The exact mechanism of this contribution is still under investigation, but there is convincing evidence that shows dietary changes accompanied by aging may promote inflammation [45, 49]. Therefore, older people’s pro-inflammatory diet might be a possible reason for the higher risk of hypothyroidism in advanced age.
Previous studies have indicated that elevated pro-inflammatory cytokines such as CRP, TNF-a, and IL-6 increase the risk of developing cardiovascular disease (CVD) in hypothyroidism [40, 50]. In other words, chronic inflammation has been known as a cause of high CVD and its mortality among hypothyroidism. The significant effect of a pro-inflammatory diet and the resultant inflammation has been seen in a higher risk of developing cardiovascular disease [51, 52]. These findings are consistent with the proposed role of diet in the occurrence of inflammation, which plays a role in hypothyroidism complications.
Further studies with a large sample size are needed to prove the relationship between DII and hypothyroidism. Future research can also study the effects of changing the inflammatory potential of diet on chronic inflammation, the risk of hypothyroidism, and its complications.
The strengths of this study could be mentioned in this case. Firstly, the enrollment of subjects with newly diagnosed hypothyroidism, as a study group, and conducted an assessment of dietary intake before they started levothyroxine treatment, which reduces the recall bias. Secondly, limited measurement errors by considering the various confounding factors such as intake of anti-inflammatory drugs and hormone replacement therapy that may modify the inflammation status in subjects. There was some limitation in this study, such as information bias due to the nature of the study which was a case-control study. Also, of 45 food parameters to calculate DII 21 items remained non-available. Another limitation could be that the type of hypothyroidism (autoimmune or non-autoimmune) was not determined, so further studies are required to identify the association of dietary inflammatory index across different types of hypothyroidism.